[EuiFlex] Add missing props to `EuiFlexGroup` and `EuiFlexItem`

[tkajtoch]

Summary

EuiFlex components are missing props for some of the regularly used flex features, like inline flex styling, flex grow and shrink, and support for the css prop.

Acceptance criteria

• EuiFlex components should support the following props
  • inline (boolean, default false) - enable display: inline-flex styling
  • grow and shrink (number, defaults to 0)
  • css - pass down to be processed by Emotion